1. Field of the Invention
The present invention relates to a bandpass filter, especially relates to a gain-boosted N-path switched-capacitor (SC) bandpass filter.
2. Description of the Prior Art
The demand of highly-integrated multi-band transceivers has driven the development of blocker-tolerant software-defined radios that can avoid the cost (and loss) of the baluns and SAW filters. The passive-mixer-first receivers achieve a high out-of-band (OB) linearity (IIP3=+25 dBm) by eliminating the forefront low-noise amplifier (LNA). However, in the absence of RF gain, a considerable amount of power is entailed for the local oscillator (LO) to drive up the mixers that must be essentially large (i.e., small on-resistance, Rsw) for an affordable noise figure (NF<5 dB). The noise-cancelling receiver breaks such a NF-linearity tradeoff, by noise-cancelling the main path via a high-gain auxiliary path, resulting in better NF (1.9 dB). However, due to the wideband nature of all RF nodes, the passive mixers of the auxiliary path should still be large enough for a small Rsw (10 Ω) such that the linearity is upheld (IIP3=+13.5 dBm). Indeed, it would be more effective to perform filtering at the antenna port.
An N-path switched-capacitor (SC) branch applied at the antenna port corresponds to direct filtering that enhances OB linearity, although the sharpness and ultimate rejection are limited by the capacitor size and non-zero Rsw that are tight tradeoffs with the area and LO power, respectively. Repeatedly adopting such filters at different RF nodes can raise the filtering order, but at the expense of power and area.
Active-feedback frequency translation loop is another technique to enhance the area efficiency (0.06 mm2), narrowing RF bandwidth via signal cancellation, instead of increasing any RC time-constant. Still, the add-on circuitry (amplifiers and mixers) penalizes the power (62 mW) and NF (>7 dB). At the expense of more LO power and noise, the output voltages can be extracted from the capacitors via another set of switches, avoiding the effects of Rsw on the ultimate rejection, but the problem of area remains unsolved.